sunriseset.c

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/*

SUNRISET.C - computes Sun rise/set times, start/end of twilight, and
             the length of the day at any date and latitude

Written as DAYLEN.C, 1989-08-16

Modified to SUNRISET.C, 1992-12-01

(c) Paul Schlyter, 1989, 1992

Released to the public domain by Paul Schlyter, December 1992

Modified to reflect minutes as well as fractional hours. 
    Curtis W. Rendon May 2003

*/


#include <stdio.h>
#include <math.h>
#include <time.h> /* so can get timezone */


/* A macro to compute the number of days elapsed since 2000 Jan 0.0 */
/* (which is equal to 1999 Dec 31, 0h UT)                           */

#define days_since_2000_Jan_0(y,m,d) \
    (367L*(y)-((7*((y)+(((m)+9)/12)))/4)+((275*(m))/9)+(d)-730530L)

/* Some conversion factors between radians and degrees */

#ifndef PI
 #define PI        3.1415926535897932384
#endif

#define RADEG     ( 180.0 / PI )
#define DEGRAD    ( PI / 180.0 )

/* The trigonometric functions in degrees */

#define sind(x)  sin((x)*DEGRAD)
#define cosd(x)  cos((x)*DEGRAD)
#define tand(x)  tan((x)*DEGRAD)

#define atand(x)    (RADEG*atan(x))
#define asind(x)    (RADEG*asin(x))
#define acosd(x)    (RADEG*acos(x))
#define atan2d(y,x) (RADEG*atan2(y,x))


/* Following are some macros around the "workhorse" function __daylen__ */
/* They mainly fill in the desired values for the reference altitude    */
/* below the horizon, and also selects whether this altitude should     */
/* refer to the Sun's center or its upper limb.                         */


/* This macro computes the length of the day, from sunrise to sunset. */
/* Sunrise/set is considered to occur when the Sun's upper limb is    */
/* 35 arc minutes below the horizon (this accounts for the refraction */
/* of the Earth's atmosphere).                                        */
#define day_length(year,month,day,lon,lat)  \
        __daylen__( year, month, day, lon, lat, -35.0/60.0, 1 )

/* This macro computes the length of the day, including civil twilight. */
/* Civil twilight starts/ends when the Sun's center is 6 degrees below  */
/* the horizon.                                                         */
#define day_civil_twilight_length(year,month,day,lon,lat)  \
        __daylen__( year, month, day, lon, lat, -6.0, 0 )

/* This macro computes the length of the day, incl. nautical twilight.  */
/* Nautical twilight starts/ends when the Sun's center is 12 degrees    */
/* below the horizon.                                                   */
#define day_nautical_twilight_length(year,month,day,lon,lat)  \
        __daylen__( year, month, day, lon, lat, -12.0, 0 )

/* This macro computes the length of the day, incl. astronomical twilight. */
/* Astronomical twilight starts/ends when the Sun's center is 18 degrees   */
/* below the horizon.                                                      */
#define day_astronomical_twilight_length(year,month,day,lon,lat)  \
        __daylen__( year, month, day, lon, lat, -18.0, 0 )


/* This macro computes times for sunrise/sunset.                      */
/* Sunrise/set is considered to occur when the Sun's upper limb is    */
/* 35 arc minutes below the horizon (this accounts for the refraction */
/* of the Earth's atmosphere).                                        */
#define sun_rise_set(year,month,day,lon,lat,rise,set)  \
        __sunriset__( year, month, day, lon, lat, -35.0/60.0, 1, rise, set )

/* This macro computes the start and end times of civil twilight.       */
/* Civil twilight starts/ends when the Sun's center is 6 degrees below  */
/* the horizon.                                                         */
#define civil_twilight(year,month,day,lon,lat,start,end)  \
        __sunriset__( year, month, day, lon, lat, -6.0, 0, start, end )

/* This macro computes the start and end times of nautical twilight.    */
/* Nautical twilight starts/ends when the Sun's center is 12 degrees    */
/* below the horizon.                                                   */
#define nautical_twilight(year,month,day,lon,lat,start,end)  \
        __sunriset__( year, month, day, lon, lat, -12.0, 0, start, end )

/* This macro computes the start and end times of astronomical twilight.   */
/* Astronomical twilight starts/ends when the Sun's center is 18 degrees   */
/* below the horizon.                                                      */
#define astronomical_twilight(year,month,day,lon,lat,start,end)  \
        __sunriset__( year, month, day, lon, lat, -18.0, 0, start, end )


/* Function prototypes */

double __daylen__( int year, int month, int day, double lon, double lat,
                   double altit, int upper_limb );

int __sunriset__( int year, int month, int day, double lon, double lat,
                  double altit, int upper_limb, double *rise, double *set );

void sunpos( double d, double *lon, double *r );

void sun_RA_dec( double d, double *RA, double *dec, double *r );

double revolution( double x );

double rev180( double x );

double GMST0( double d );



/* A small test program */

main()
{
      int year,month,day;
      double lon, lat;
      double daylen, civlen, nautlen, astrlen;
      double rise, set, civ_start, civ_end, naut_start, naut_end,
             astr_start, astr_end;
      double daylen_int,daylen_frac;  /* CWR */
      int    rs, civ, naut, astr;
      int daylen_hours,daylen_min, gmtoff_hr; /* cwr */
      double rise_int,rise_frac,set_int,set_frac; 
      int rise_hours,rise_min,set_hours,set_min;

      time_t the_time; /* only need so can get time zone of local machine... */
      struct tm *mylocal_time;
      char buf[80];
      
/* get time and offset from GMT */ 
      time(&the_time);
      mylocal_time = localtime(&the_time);
      gmtoff_hr = mylocal_time->tm_gmtoff/3600;

      printf("Enter longitude and latitude in decimal degrees, i.e. -97.75 30.23\n");
      printf("Hint: divide minutes by 60 to get decimal portion\n");
      printf( "Longitude (+ is east) and latitude (+ is north) : " );
      fgets(buf, 80, stdin);
      sscanf(buf, "%lf %lf", &lon, &lat );

      for(;;)
      {
            printf( "Input date ( yyyy mm dd ) (ctrl-C exits): " );
            fgets(buf, 80, stdin);
            sscanf(buf, "%d %d %d", &year, &month, &day );

            daylen  = day_length(year,month,day,lon,lat);
            civlen  = day_civil_twilight_length(year,month,day,lon,lat);
            nautlen = day_nautical_twilight_length(year,month,day,lon,lat);
            astrlen = day_astronomical_twilight_length(year,month,day,
                  lon,lat);
                  
            daylen_frac = modf(daylen,&daylen_int); /* daylen into int and fraction */
            daylen_hours = (int)daylen_int;
            daylen_min = (int)(daylen_frac * 60);
            printf( "Day length:                 %5.2f hours", daylen );
            printf( "(%2d hours %2d min)\n", daylen_hours,daylen_min );
            daylen_frac = modf(civlen,&daylen_int); 
            daylen_hours = (int)daylen_int;
            daylen_min = (int)(daylen_frac * 60);
            printf( "With civil twilight         %5.2f hours", civlen );
            printf( "(%2d hours %2d min)\n", daylen_hours,daylen_min );
            daylen_frac = modf(nautlen,&daylen_int); 
            daylen_hours = (int)daylen_int;
            daylen_min = (int)(daylen_frac * 60);
            printf( "With nautical twilight      %5.2f hours", nautlen );
            printf( "(%2d hours %2d min)\n", daylen_hours,daylen_min );
            daylen_frac = modf(astrlen,&daylen_int); 
            daylen_hours = (int)daylen_int;
            daylen_min = (int)(daylen_frac * 60);
            printf( "With astronomical twilight  %5.2f hours", astrlen );
            printf( "(%2d hours %2d min)\n", daylen_hours,daylen_min );
            daylen_frac = modf((civlen-daylen)/2.0,&daylen_int); 
            daylen_hours = (int)daylen_int;
            daylen_min = (int)(daylen_frac * 60);
            printf( "Length of twilight: civil   %5.2f hours",
                  (civlen-daylen)/2.0);
            printf( "(%2d hours %2d min)\n", daylen_hours,daylen_min );
            daylen_frac = modf((nautlen-daylen)/2.0,&daylen_int); 
            daylen_hours = (int)daylen_int;
            daylen_min = (int)(daylen_frac * 60);
            printf( "                  nautical  %5.2f hours",
                  (nautlen-daylen)/2.0);
            printf( "(%2d hours %2d min)\n", daylen_hours,daylen_min );
            daylen_frac = modf((astrlen-daylen)/2.0,&daylen_int); 
            daylen_hours = (int)daylen_int;
            daylen_min = (int)(daylen_frac * 60);
            printf( "              astronomical  %5.2f hours",
                  (astrlen-daylen)/2.0);
            printf( "(%2d hours %2d min)\n", daylen_hours,daylen_min );

            rs   = sun_rise_set         ( year, month, day, lon, lat,
                                          &rise, &set );
            civ  = civil_twilight       ( year, month, day, lon, lat,
                                          &civ_start, &civ_end );
            naut = nautical_twilight    ( year, month, day, lon, lat,
                                          &naut_start, &naut_end );
            astr = astronomical_twilight( year, month, day, lon, lat,
                                          &astr_start, &astr_end );

            daylen_frac = modf(((rise+set)/2.0)+gmtoff_hr ,&daylen_int); 
            daylen_hours = (int)daylen_int;
            daylen_min = (int)(daylen_frac * 60);

            printf( "Sun at south %5.2fh UT,%20d:%02d local\n", (rise+set)/2.0,daylen_hours,daylen_min);

            switch( rs )
            {
                case 0:
                    /* add minutes and local. CWR*/
                    rise_frac = modf(rise+gmtoff_hr ,&rise_int); 
                    rise_hours = (int)rise_int;
                    rise_min = (int)(rise_frac * 60);
                    set_frac = modf(set+gmtoff_hr ,&set_int); 
                    set_hours = (int)set_int;
                    set_min = (int)(set_frac * 60);

                    printf( "Sun rises %5.2fh UT, sets %5.2fh UT;\n    rises %02d:%02d,     sets %02d:%02d local\n",
                             rise, set, rise_hours,rise_min,set_hours,set_min);
                    break;
                case +1:
                    printf( "Sun above horizon\n" );
                    break;
                case -1:
                    printf( "Sun below horizon\n" );
                    break;
            }

            switch( civ )
            {
                case 0:
                    rise_frac = modf(civ_start+gmtoff_hr ,&rise_int); 
                    rise_hours = (int)rise_int;
                    rise_min = (int)(rise_frac * 60);
                    set_frac = modf(civ_end+gmtoff_hr ,&set_int); 
                    set_hours = (int)set_int;
                    set_min = (int)(set_frac * 60);

                    printf( "Civil twilight starts %5.2fh, "
                            "ends %5.2fh UT;\n               starts %02d:%02d,  ends %02d:%02d local\n", 
                            civ_start, civ_end, 
                            rise_hours, rise_min, set_hours, set_min);
                    break;
                case +1:
                    printf( "Never darker than civil twilight\n" );
                    break;
                case -1:
                    printf( "Never as bright as civil twilight\n" );
                    break;
            }

            switch( naut )
            {
                case 0:
                    rise_frac = modf(naut_start+gmtoff_hr ,&rise_int); 
                    rise_hours = (int)rise_int;
                    rise_min = (int)(rise_frac * 60);
                    set_frac = modf(naut_end+gmtoff_hr ,&set_int); 
                    set_hours = (int)set_int;
                    set_min = (int)(set_frac * 60);

                    printf( "Nautical twilight starts %5.2fh, "
                            "ends %5.2fh UT;\n                  starts %02d:%02d,  ends %02d:%02d local\n", 
                            naut_start, naut_end, 
                            rise_hours, rise_min, set_hours, set_min );
                    break;
                case +1:
                    printf( "Never darker than nautical twilight\n" );
                    break;
                case -1:
                    printf( "Never as bright as nautical twilight\n" );
                    break;
            }

            switch( astr )
            {
                case 0:
                    rise_frac = modf(astr_start+gmtoff_hr ,&rise_int); 
                    rise_hours = (int)rise_int;